1. Field of the Invention
An aspect of the present invention relates to a method for assembling an array-type semiconductor laser device and, more particularly, method for assembling an array-type semiconductor laser device used as a light source of a laser beam printer, a laser copying machine, or the like and having a plurality of light emission spots aligned in such a way that an interval between laser light sources is at a narrow interval of 30 μm or less.
2. Description of the Related Art
In order to speed up the laser beam printer, it is effective to scan a plurality of laser beams simultaneously. The array-type semiconductor laser, in which a plurality of laser resonators are integrated in a semiconductor laser chip, enables a single optical system to scan a plurality of beams and thus is a useful device as the light source for the high-speed printer.
Normally, the semiconductor laser is formed by growing epitaxially the so-called “double hetero structure”, in which a semiconductor layer with a narrow band gap is put between semiconductor layers having a different conductivity and a wide band gap respectively, on the principal plane of the semiconductor substrate crystal. The heat generation when a current is supplied in operating the laser is caused mainly in this portion of the double hetero structure. Therefore, in order to radiate the heat effectively, the side surface of the laser chip, on which the double hetero structure is formed, must be bonded onto the plate material that is called the submount and is formed of the high heat-conductive insulator. Such semiconductor laser assembling approach is called the “junction down assembling”.
In the case of the semiconductor laser for the printer, it is requested that an operating current must be suppressed low and a laser beam must be radiated in a fundamental lateral mode. Therefore, the double hetero structure must be provided in the direction perpendicular to the substrate plane and also the stripe-like waveguide structure for propagating only in the direction perpendicular to the traveling direction of the laser beam on the plane that is coplanar to the substrate plane must be provided. In the case of the array-type semiconductor laser, it is needed that a plurality of such stripe-like waveguide structures should be provided in a single laser chip and be caused to each oscillate the laser beam independently. In the junction down assembling, it is difficult to provide the channels at a narrow interval of 30 μm or less because short-circuit may arise between the neighboring electrodes of the stripe-like waveguide structures when the submount and the laser chip are soldered.
As the approach of scanning the neighboring scanning lines, i.e., the scanning lines arranged at the same interval as the size of the light spot, by the laser array of a wide-interval beam spots, such a system is employed that the line along which spots of the array-type semiconductor laser are aligned is scanned by inclined slightly with respect to the direction of the scanning line. However, since an inclination of the alignment line of the laser array from the scanning line is minute, an angular control is difficult in this system. For this reason, the laser array of a narrow-interval beam spots is needed.
Also, in order to implement the higher printing speed, an increase of the channel number (e.g., 20 channels or more) is required of the array-type semiconductor laser. In the case where the channel interval is 30 μm or more, the light emission spots are interspersed along a range of 600 μm. As a result, it is difficult to converge all light emission spots into a good shape by a single optical system and also keep a parallelism of the scanning lines.
JP-2006-24665-A discloses the array-type semiconductor laser having the upper surface structure shown in FIG. 1 and the sectional structure shown in FIG. 2. FIG. 2 is a sectional view taken along an A-A′ line in FIG. 1. In this structure, stripe-like waveguide structures 1 each constituting the resonator of the semiconductor laser, first electrodes 2 provided in the direction parallel with the stripe-like waveguide structures 1 to cover the structures 1 and separated electrically, and an insulating film 3 for covering them are formed, then holes 4 for supplying a power are opened in a part of the insulating film 3, then second electrodes 5 split in the direction intersecting with the stripe-like waveguide structures are provided, and then submount electrodes 9 each having a different shape from the second electrodes 5 and formed of an electrode layer 7 and a solder layer 8 are provided on a submount 6 shown in FIG. 3. When the second electrodes 5 and the submount electrodes 9 having such structures are joined with solder, alignment accuracy in the junction down assembling is decided depending on the interval of the second electrodes 5. Therefore, the channel interval of the array laser can be narrowed up to the limit of the lithography technology, while the heat radiating characteristic can be maintained good because the heat generated in the stripe-like waveguide structure portions when a power is fed is radiated from the second electrodes 5 to the submount 6 via the thin insulating film 3.
In FIG. 1, FIG. 2, and FIG. 3, a reference 101 denotes an n-type GaAs substrate, a reference 102 denotes an n-type (Al0.7Ga0.3)0.5In0.5P cladding layer, a reference 103 denotes a multiple quantum well active layer whose number of wells is 3, a reference 104 denotes a p-type (Al0.7Ga0.3)0.5In0.5P cladding layer, and a reference denotes a p-GaAs cap layer 105. The multiple quantum well active layer 103 is constructed by a Ga0.3In0.5P strained quantum well layer (undoped) 106, and a (Al0.5Ga0.5)0.5In0.5P barrier layer (undoped) 107 of 5 nm thickness.
As the approach of executing a precise alignment while preventing the short-circuit of the array-type semiconductor laser, JP-H06-334274-A discloses that projection portions 12 of laser chips 11 are fitted into a recess-like matching structure 10 that is provided to the submount 6 and extended in the same direction as the stripe-like waveguide structure 1, as shown in FIG. 4. The structure has features that the position of the stripe-like waveguide structure 1 can be decided with good accuracy by the physical fitting of the projection portions 12 into the recess-like matching structure 10 and the laser arrays provided separately and having the different characteristic respectively can be aligned with good interval accuracy.
According to JP-2006-24665-A, the laser array whose channel interval is narrowed up to the limit of the lithography technology can be implemented. In this case, the second electrodes 5 are split in the length direction of the stripe-like waveguide structure 1, and thus the number of arrays is restricted by the interval of the second electrodes. Since the optimum value necessary for the compatibility between a low operating current and high reliability exists in the length of the waveguide of the laser array, it is difficult to adjust a length of the resonator in accordance with the number of arrays. In the case of this structure, the alignment in the direction intersecting orthogonally with the waveguides is also needed to control positions of the light emission spots, and the two-directional adjustment is required.
Meanwhile, according to JP-H06-334274-A, a plurality of array-type lasers can be aligned with good accuracy. In this case, the alignment accuracy is high but position control in the fitting operation is difficult when a play between the recess and the projection is small, while the alignment in the fitting operation becomes easy but there existed such a problem that an error occurs in positional accuracy in the completed structure when a play between the recess and the projection is large. Also, when the interval of the array lasers is narrow, the projection portions of the laser chip run up onto the projection portions of the submount in the fitting operation on account of displacement between the chip and the submount in the rotational direction. Therefore, not only the fitting operation becomes difficult but also a part of the laser chip, especially a part of the stripe-like waveguide structure 1, is sometime damaged.